There are two types of muscle fibers, fast twitch (FT) and slow twitch (ST). Fast twitch fibers are used for explosive type movements and are easily fatigued. Slow twitch muscle fibers contain more mitochondria than Fast twitch. Mitochondria are cell structures that contain specific enzymes, which are required by the cell in order to use oxygen for energy production.

Fast twitch muscles fibers have less mitochondria and therefore less capacity for oxygen utilization in the production of energy within the muscle. This makes them better suited to anaerobic activities such as weight training, sprinting, jumping and other explosive type activities. FT fibers create energy anaerobically, that is, without oxygen. This system uses glucose as a prime energy source. The by-product of this anaerobic energy production is heat and lactic acid. Lactic acid accumulation in the muscle causes fatigue and soreness. The anaerobic energy system is a limited system for energy production.

Slow twitch fibers are used for endurance type activities and are particularly suited to aerobic type activities. These type fibers contain an increased number of mitochondria and therefore are capable of utilizing oxygen for the production of energy within the muscle. This system uses glucose or fat in combination with oxygen to produce energy. The by-product of this system is carbon dioxide, water and heat.

Each person has a specific ratio of FT to ST fibers. A person with a high ratio of FT fibers may find it easier to train for specific activities that involve explosive movements. Conversely, a person with a higher ratio of ST fibers might find it easier to train and excel at endurance type activities.

There is a third type of muscle fiber that exists only in humans. It is considered a FT fiber of type IIA. These fibers are less powerful than the type IIAB discussed above. What makes these type IIA FT fibers unique is that they can adapt somewhat to aerobic activities. These fibers provide the capability to alter our original genetic FT/ST ratio.

Energy Production

There are basically two types of energy systems that the body utilizes, Aerobic and Anaerobic.
Each energy system produces Adenosine Triphosphate (ATP), which is used by the muscles to contract.

The Aerobic System can utilize carbohydrates, proteins or fat to supply an unlimited amount of ATP as long as oxygen is present. The Aerobic system provides medium to very long duration energy production with low to moderate power (less than 85% of maximum output). The by-product of this system is heat, water and carbon dioxide.

The Anaerobic System can only utilize carbohydrates for ATP production. This system does not use oxygen in the metabolization of it's fuel source. The Anaerobic System provides short duration (45 - 70 seconds) and high power. The by-product of the metabolization of glucose (glycolysis) in this system is heat and lactic acid, the cause of muscle soreness immediately after exercise. Muscle soreness 24 to 48 hours after exercise is due to torn muscle fibers and connective tissue. This type of soreness can be reduced by adequate warm-up and cool-down stretching exercises.

Aerobic capacity is the ability of the body to collect and transfer oxygen from the air through the lungs and blood to the working muscles. This is related to cardio-respiratory endurance and is referred to as Maximal Oxygen Consumption or VO2 max. Aerobic Capacity reduces at about 10% per decade after 30 years of age.

The Anaerobic Threshold is defined as that point where the body can no longer meet the oxygen demand and it's anaerobic metabolism is accelerated. This point varies on an individual basis and is dependent on fitness level. For healthy individuals, this occurs between 50% and 66% of their maximal working capacity. This would be equivalent to running faster than half speed.

Cardiovascular and Respiratory System

Air is inhaled into the lungs where oxygen is exchanged through tiny gas permeable sacs within the lungs for carbon dioxide from the blood. The heart pumps the oxygen rich blood from the left atrium through the arteries then through tiny vessels called capillaries to the tissues of the body. At the cell level, oxygen is given up for metabolism and the carbon dioxide produced by this action is picked up by the blood. The oxygen depleted and carbon dioxide rich blood is then pumped back to the heart, through the veins to the right atrium to the lungs where the process is repeated.

Aerobic activity increases the strength of the heart muscle. The result is a greater volume of blood per stroke. This is referred to as Stroke Volume or the amount of blood ejected from each ventricle of the heart during one stroke. Cardiac Output is a measure of the amount of blood pumped through each ventricle in one minute. Vital Capacity is the volume of air that can be forcibly ejected from the lungs in a single expiration. Aerobic activity provides a Training Effect on Vital Capacity, Stroke Volume and Cardiac Output.

An artery carries blood away from the heart while a vein carries blood towards the heart.